Flexible pipes or hoses are used in both onshore and offshore applications in the field of oil & gas exploration primarily for the transportation of fluids and gases. The term “flexible” is to be understood to exclude substantially rigid constructions, such as steel pipes. Flexible pipes or hoses are typically used to transport pressurised, high temperature crude oil and gas from a seabed based wellhead to a floating platform or processing facility. However, they are equally suitable for carrying a wide range of injection or service fluids aimed at enhancing or maintaining production output. Consequently, flexible pipes hoses are exposed to wide variations in both internal & external pressure and temperature.
Typical pipe or hose pressure and temperature ratings are disclosed in, e.g., American Petroleum Institute (API) standard 7K—5th Edition—June 2010, tiled ‘Drilling and Well Servicing Equipment’ (see paragraphs 9.6.1, 9.6.3.1 and Table 9). Further guidance on the pressure and/or temperature conditions experienced in flexible pipes or hoses (typically of diameter of 3″ and above) can be found in API standards, 16C (see Tables 3.4.1, 3.4.2, 3.4.3 and 3.5.2.1), OCIMF, 17J and 17K. In moderate diameter hoses the pressure rating will typically be several hundred bar (e.g. of the order of 10 to over 100 MPa.), and will decrease with increasing hose diameter. Flexible pipes or hoses must be able to withstand typical temperature conditions of approximately −40° C. to +132° C. depending upon the application.
Flexible pipes can be divided into two categories: bonded and unbonded. Unbonded flexible pipes typically comprise a number of metallic armour layers and polymeric anti-wear/anti-friction layers whereby a degree of relative slippage between at least two adjacent layers is possible. Even where one layer is embedded within another, the non-bonded characteristics can be demonstrated by a simple pull out test of, for example, a steel cord reinforcement layer out of its surrounding polymer matrix. This test is based on an adapted version of the ‘Standard Test Method for Rubber Property—Adhesion to Steel Cord’ (ASTMD 2229-85)’.
Bonded pipes—which are commonly used for a range of similar applications as non-bonded pipes—preclude any slippage between adjacent layers. Bonded pipes employ either an inner elastomeric or thermoplastic liner pipe that is sometimes extruded on or bonded to an underlying metal carcass and surrounded by an armour layer. The entire pipe can be considered to be a composite of bonded metal cable or wire (either brass-coated or galvanised) and elastomeric layers, with the possible inclusion of a thermoplastic inner liner layer as noted below. Bonded pipes are tested to ensure that they are capable of withstanding rapid gas decompression events which can cause a blistering phenomenon in their innermost layers. Bonded flexible pipe types can be used as flexible risers, loading hoses or hoses for exploration (rotary hoses, choke & kill hoses, mud & cement hoses).
In order to avoid liquid and gas permeation losses, a thermoplastic pipe—or a pipe having a thermoplastic inner liner—is often employed so as to enhance its overall sealing ability. However, some fluids or gases can be very aggressive and cause rapid degradation of certain polymers or plastics liners, especially at higher temperatures. To address this problem, it is known to employ more chemically inert polymer types within the pipe and/or its inner liner (if present) such as cross-linked polyethylene (PEX), polyamide 11/12 (PA11/PA12), ethylene tetrafluoroethylene (ETFE), polyvinylidene fluoride (PVDF) or polyether ether ketone (PEEK). Whilst these engineered polymers offer many advantages in terms of temperature resistance, sour corrosion resistance, anti-cracking behaviour, low gas/liquid permeability etc., they are known to be less effective in ensuring a good quality seal with a hose coupling (i.e. either a pipe-to-pipe or pipe-to-end-fitting connection). Often these liners are reinforced with an inner (stainless steel) carcass.
To ensure a reliable seal between a pipe, or an inner liner thereof, and a coupling (i.e. either a pipe-to-pipe or pipe-to-end-fitting connection) all ‘leakage paths’ must be eliminated. A leakage path is any path allowing pressurized fluid or gas to equalise with a lower pressure state. Fabrics, air pockets, armouring cables or any continuous bonding failure between adjacent layers or materials within the pipe construction can all give rise to such a ‘leakage path’ causing localised gas/liquid accumulations leading to blistering and, ultimately, failure of the flexible pipe. A secure and effective seal is a fundamental safety requirement for high pressure, high temperature bonded pipes. To the best knowledge of the inventor, all manufacturers of bonded hoses have to date employed an elastomer-based sealing compound, e.g. such as that disclosed in GB2329439B (Antal et al).
Elastomers constitute the most flexible, deformable and elastic of the three classes of the non-metallic polymer materials. A behavioural characteristic of all elastomers is that they are inherently permeable to gases and vapours. When used within a flexible pipe for high temperature/high pressure applications, transmission of dissolved gases into microvoids within the elastomeric structure make the pipe more prone to rupture during a rapid depressurisation event, i.e. whereby bubbles form within the microvoids when pressure externally of the pipe is lost. This type of pipe failure is known as ‘explosive decompression’ and results in a catastrophic failure of the pipe seal and/or lining.
In an effort to minimise instances of ‘explosive decompression’, flexible pipes for high temperature/high pressure applications typically employ an inner thermoplastic liner to reduce the likelihood of liquid/gas permeation; and an inner strip-wound steel carcass located radially inside the thermoplastic liner to reduce the likelihood of blistering. Whilst such preventative measures are generally effective along the length of a pipe, the sealed connection between a pipe and its coupling (i.e. either a pipe-to-pipe or pipe-to-end-fitting connection) is an area which remains the subject of sealing stresses. Such stresses arise from mechanically applied compression and/or compression arising from the hydrostatic pressure of the fluid being sealed.
In terms of their functional properties, elastomers are soft, substantially elastic, and substantially incompressible. Such characteristics make elastomers suitable for use as primary seals at the interface of a pipe and its coupling. The inherent incompressibility of elastomers means that high stresses can be resisted, and high pressures can be accommodated, when the elastomer material is highly constrained. Elastomeric seals are therefore an automatic choice for high temperature/high pressure applications. Nevertheless, several modes of elastomeric failure or deterioration are well documented, as summarised in Tables 1 and 2 in the Health & Safety Executive Report No. 320 (2005) titled: ‘Elastomers for fluid containment in offshore oil and gas production: Guidelines and review’ (ISBN 0 7176 2969 4). In Table 1, a failure mode of “Rapid gas decompression or explosive decompression (ED)” is described as follows: “Gas dissolved in the elastomer under high pressure conditions comes out of solution and forms bubbles in the material when the external pressure is lost. The bubbles may grow sufficiently to cause fracture of the material (e.g. seals) or of an interface (e.g. between the liner and adjacent layer in a hose).
In view of the known modes of elastomeric failure or deterioration, only one of which is described above, the present inventor has concluded that there is a requirement for alternative flexible pipe arrangements providing a recognisable improvement over current sealing performance whilst simplifying overall pipe construction and methods of manufacture. In particular, further improvements in terms of the ability of a flexible pipe or hose and any associated coupling to withstand hostile conditions, over a longer period, would be highly desirable.